Wednesday, September 3, 2008

4:00 pm - 5:00 pm

Electrochemical deposition of magnetic nanowire arrays in porous Al2O3 templates (Seminar) Speaking: Zuxin Ye Abstract: Magnetic nanowire arrays are highly desirable for both fundamental research and applications, in fields such as ultracold atoms and superconductivity. Electrochemical deposition of nanowires into porous aluminum oxide (AAO) templates is a low-cost and efficient technique for growing high-density magnetic nanowire arrays with an ultrahigh nanowire aspect ratio, tunable nanowire diameters and large sample sizes. Two studies about the nanowire array synthesis and structure characterization will be discussed. Electroplated nanowires have complicated microstructure due to the size confinement by the AAO pores and the electrochemical deposition processes. The magnetic properties are strongly influenced by the microstructure. We investigated the correlation between the crystal structure and the magnetic properties of Co nanowire samples of two typical nanowire diameter values, 65 and 200 nm. Strikingly different microstructure has been observed. The 65 nm-thick Co nanowires are composed of long Co single crystal segments with a hexagonal close-packed major phase, while the 200 nm-thick Co nanowires are composed of submicron-long hexagonal close-packed and face-centered cubic Co single crystallite segments. Correspondingly, different magnetic properties have been observed for these Co nanowires. The 65 nm-thick Co nanowires have a magnetic hysteresis that is significantly larger than that of the 200 nm-thick Co nanowires. Spontaneous magnetic moments of the nanowires are parallel to the nanowires in the 65 nm-thick Co nanowires, but they are transverse to the nanowires in the 200 nm-thick Co nanowires. In the applications of nanowire arrays, the independent control of geometric parameters, such as the diameter and the center-to-center spacing of the nanowires, is of critical importance. The geometric parameters are controlled by the pore diameter and the pore spacing of the AAO templates. It is virtually impossible to independently tune these two parameters during the growth of AAO membranes, since they are coupled to each other and are both nearly proportional to the applied anodization voltage. We demonstrated a simple and efficient way for reducing the pore diameter and independently tuning the geometric parameters of nanowire arrays by depositing first an array of insulating oxide nanotubes and then magnetic nanowires, into the AAO pores. Location: ENPH 501 Sponsor: I. Lyuksyutov

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